Showcase is an extension thought to easily locate and select any open
browser window in Firefox.

You can use it in two ways: global mode (F12) or local mode (Shift +
F12). In global mode, a new window will be opened with thumbnails of
the browsers you've opened in all windows. In local mode, only tabbed
browsers of your current window will be shown.

WWW: http://showcase.uworks.net/